This invention relates to an improved apparatus for crystallization processes and more specifically to a type of crystallizer having a plurality of vibrating perforated plates at intervals along the crystallizer length.
A major problem in carrying out industrial crystallizations by indirect heat transfer from saturated solutions is the phenomenon called incrustation. Incrustation is the deposition of precipitated or crystalline solids on the surface through which heat is transferred from the saturated solution to the cooling fluid. Such incrustations reduce the rate of heat transfer and make necessary frequent shutdowns for cleaning of equipment.
Designers of industrial crystallizers have attempted to abate incrustations by providing scrapers to continuously remove solid deposits from the heat transfer surface. A typical design comprises a set of horizontal jacketed pipes each having a centrally located rotating shaft with peripheral scrapers. The solution to be crystallized is pumped through the inner pipe while cooling fluid is concurrently pumped through or vaporized in the annulus. Heat is extracted from the solution, crystals are formed, and incrustations develop on the wall of the inner pipe. The rotating scrapers remove the incrustations from the heat transfer surface.
The scrapers have proven to be somewhat effective by increasing run duration from a few minutes or hours to one to fourteen or more days. However, at commercially acceptable heat fluxes the scrapers and rotating shafts themselves become incrusted. The incrustation can become so thick that the inner pipe becomes plugged or the rotating members are damaged. Before this happens it is necessary to shut down the crystallizer and clean the inner pipe by heating it to melt the solid deposits, by washing it with solvent or by manually scraping it.
Other commercially available crystallizers include a set of internally cooled plates disposed in a vertical or horizontal tank. Such designs usually include a rotating shaft to which wipers are attached. The wipers are positioned so that the surfaces of the plates are wiped as the shaft rotates. However, if the temperature differential is increased to provide a good production rate, the shaft, wipers or plates rapidly become incrusted requiring an interruption in operation to melt off the incrustations.
Attempts at improving conventional designs have further comprised heating the scrapers with electric or hot fluid tracers while the scrapers are rotating. Although this can be somewhat effective, in doing so one usually puts a great deal of heat back into the solution to be cooled and thus limits the capacity of the equipment in addition to increasing operating cost. In any event, the effective installation of heaters on mechanically complicated scrapers is difficult and expensive.
Heat transfer surfaces in crystallizers can be kept clean by inducing ultrasonic vibrations in them. This method does in fact work well in laboratory apparatus. Unfortunately, no way has been found to scale up ultrasonic crystallizer units to a commercially acceptable capacity.
The rate of formation of incrustations generally increases rapidly as the difference in temperature between the solution to be crystallized and the cooling fluid increases. As a practical matter, operators of crystallizers generally limit the temperature differential to a magnitude at which they are able to get a fairly long run time between cleanings. However, operating at low temperature differentials requires relatively large surface areas and correspondingly large capital investment to provide commercially acceptable capacity. If higher temperature differentials could be used, there would often be a substantial reduction in crystallizer capital cost
British patent No. 1,365,536 discloses a countercurrent crystallizer apparatus which comprises individual crystallization, purification, and melt sections. The purification section comprises a plurality of perforated plates positioned at spaced intervals in a cylindrical enclosure so that the crystal mass may pass the plates countercurrently to the mother liquor. Free moving bodies such as spheres are placed on each perforated plate. The spheres are set in motion by vibrating the entire column, vibrating the set of perforated plates or by other means. Although this invention does increase the purification efficiency, it still suffers from the short run time chracteristic of conventional crystallizers because this invention incorporates conventional chillers to generate the crystal crop that is subsequently purified in the above-described purification section.
It is the overall object of the present invention to provide an improved apparatus for making crystals by removing heat from a solution or slurry.
Accordingly, it is an object of the present invention to provide a crystallizer apparatus with high heat transfer coefficients relative to prior art apparatus.
It is another object of the present invention to provide a crystallizer apparatus capable of long run times between cleanings.
It is yet another object of the present invention to provide an efficient crystallizer column which can be economically scaled up to commercial size.
These and other objects and advantages will become apparent to those skilled-in the art from the following description and figures.